At present, a development trend of the submarine cable communications industry is that: communication between a terminal-station device and an underwater device, for example, communication between a terminal-station device and an underwater delay device or an underwater splitting device, needs to be implemented; therefore, a working state, a performance indicator, a fault situation, information about switching between an electric circuit and an optical circuit, and the like, that are of the underwater device are learned, so that network routing and switching, fault locating, fault processing and clearing, and the like are performed.
Referring to FIG. 1, FIG. 1 is a schematic diagram of communication between a terminal-station device and an underwater device. A terminal-station device 1a delivers a command (command signal) to at least one underwater device 1b, to notify the at least one underwater device 1b to receive the command and perform an action. After receiving the command, the selected underwater device 1b feeds back, according to a requirement of the terminal-station device 1a, device performance, a working state, whether to switch an electric circuit or optical circuit, and the like, thereby implementing intelligent monitoring between the terminal-station device 1a and the underwater device 1b. 
Referring to FIG. 2, FIG. 2 is a structural diagram of an underwater device in the prior art. The underwater device shown in FIG. 2 uses a method of modulated optical output power, to implement communication with a terminal-station device.
As shown in FIG. 2, 20a is an uplink optical path, 40a is a downlink optical path, 23a is a pump unit, 21a is an erbium-doped fiber (EDF), 27a is an optical isolator, 25a is an optical multiplexer (wavelength division multiplexer), and 29a is a coupler. The bump unit 23a includes: 60a, which is a pump laser; 64a, which is a pump controller (pump controller); and 65a, which is a supervisory processor.
The underwater device shown in FIG. 2 enables, by adjusting the pump laser 60a, a peak modulation signal with respect to output power of an amplifier. Specifically, the pump controller 64a changes a working current of the pump laser 60a, and modulates output power of the pump laser 60a, to make input power of the erbium-doped fiber 21a change, thereby modulating output power of an optical amplifier, and implementing the modulated optical output power.
In the prior art, the output power of the pump laser 60a is modulated on a primary optical path, so as to implement modulated optical output power of an uplink optical path and a downlink optical path. The technical solution has the following disadvantages:
(1) Because the total power on the primary optical path is modulated, a service will be affected once a fault occurs, and service interruption may be caused in a serious situation, which does not meet a high reliability requirement of an underwater device; and
(2) A special light emitting device, the pump laser 60a, is required, and the pump laser 60 is expensive and prone to damages, resulting in high costs and difficult maintenance.